Glutamate is the major amino-acid transmitter in the mammalian central nervous system (CNS). Glutamate plays a major role in numerous physiological functions, such as learning and memory but also sensory perception, development of synaptic plasticity, motor control, respiration and regulation of cardiovascular function. Furthermore, glutamate is at the center of several different neurological and psychiatric diseases, where there is an imbalance in glutamatergic neurotransmission.
Glutamate mediates synaptic neurotransmission through the activation of ionotropic glutamate receptor channels (iGluRs), namely the NMDA, AMPA and kainate receptors which are responsible for fast excitatory transmission (Nakanishi et al., (1998) Brain Res Rev., 26:230-235).
In addition, glutamate activates metabotropic glutamate receptors (mGluRs) which have a more modulatory role that contributes to the fine-tuning of synaptic efficacy.
The mGluRs are G protein-coupled receptors (GPCRs) with seven-transmembrane spanning domains and belong to GPCR family 3 along with the calcium-sensing, GABAb and pheromone receptors.
The mGluR family is composed of eight members. They are classified into three groups (group I comprising mGluR1 and mGluR5; group II comprising mGluR2 and mGluR3; group III comprising mGluR4, mGluR6, mGluR7 and mGluR8) according to sequence homology, pharmacological profile and nature of intracellular signalling cascades activated (Schoepp et al. (1999) Neuropharmacology, 38:1431-76).
Glutamate activates the mGluRs through binding to the large extracellular amino-terminal domain of the receptor, herein called the orthosteric binding site. This activation induces a conformational change of the receptor which results in the activation of the G-protein and intracellular signalling pathways.
In the central nervous system, mGluR4 receptors are expressed most intensely in the cerebellar cortex, basal ganglia, sensory relay nuclei of the thalamus and hippocampus (Bradley et al. (1999) Journal of Comparative Neurology, 407:33-46; Corti et al. (2002) Neuroscience, 110:403-420). The mGluR4 subtype is negatively coupled to adenylate cyclase via activation of the Gαi/o protein, is expressed primarily on presynaptic terminals, functioning as an autoreceptor or heteroceptor and activation of mGluR4 leads to decreases in transmitter release from presynaptic terminals (Corti et al. (2002) Neuroscience, 110:403-420; Millan et al. (2002) Journal of Biological Chemistry 277:47796-47803; Valenti et al. (2003) Journal of Neuroscience 23:72218-7226).
Orthosteric agonists of mGluR4 are not selective and activate the other Group III mGluRs (Schoepp et al. (1999) Neuropharmacology 38:1431-1476). The Group III orthosteric agonist L-AP4 was able to reduce motor deficits in animal models of Parkinson's disease (Valenti et al. (2003) J. Neurosci. 23:7218-7226) and decrease excitotoxicity (Bruno et al. (2000) J. Neurosci. 20; 6413-6420) and these effects appear to be mediated through mGluR4 (Marino et al. (2005) Curr. Topics Med. Chem. 5:885-895). In addition to LAP-4, ACPT-1, another selective group III mGluR agonist has been shown to caused a dose-and-structure dependant decrease in haloperidol-induced catalepsy and attenuated haloperidol-increased Proenkephalin mRNA expression in the striatum (Konieczny et al. 2007). Furthermore, Lopez et al. (2007, J Neuroscience) have shown that bilateral infusions of ACPT-I or LAP-4 into the globus pallidus fully reversed the severe akinetic deficits produced by 6-hydroxydopamine lesions of nigrostriatal dopamine neurons in a reaction-time task without affecting the performance of controls. In addition, the reversal of haloperidol-induced catalepsy by intrapallidal ACPT-1 was prevented by concomitant administration of a selective group III receptor antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine. The opposite effects produced by group III mGluR activation in the SNr strongly suggest a role of mGluR4 rather than others mGluR receptor sub-types in normalizing basal ganglia activity (Lopez et al. 2007).
These results suggest that, among mGluRs subtypes, mGluR4 is believed to be the most interesting novel drug target for the treatment of Parkinson's disease (for a review see Conn et al. Nature Review Neuroscience 2005).
Symptoms of Parkinson's disease appear to be due to an imbalance in the direct and indirect output pathways of the basal ganglia and reduction of transmission at the inhibitory GABAergic striato-pallidal synapse in the indirect pathway may result in alleviation of these symptoms (Marino et al. (2002) Amino Acids, 23:185-191).
mGluR4 is more abundant in striato-pallidal synapses than in striato-nigral synapses, and its localization suggests function as a presynaptic heteroceptor on GABAergic neurons (Bradley et al. (1999) Journal of Comparative Neurology, 407:33-46) suggesting that selective activation or positive modulation of mGluR4 would decrease GABA release in this synapse thereby decreasing output of the indirect pathway and reducing or eliminating the Parkinson's disease symptoms.
A new avenue for developing selective compounds acting at mGluRs is to identify molecules that act through allosteric mechanisms, modulating the receptor by binding to a site different from the highly conserved orthosteric binding site.
Positive allosteric modulators of mGluRs have emerged recently as novel pharmacological entities offering this attractive alternative. This type of molecule has been discovered for mGluR1, mGluR2, mGluR4, mGluR5, mGluR7 and mGluR8 (Knoflach F. et al. (2001) Proc. Natl. Acad. Sci. USA., 98:13402-13407; Johnson K et al. (2002) Neuropharmacology, 43:291; O'Brien J. A. et al. (2003) Mol. Pharmacol., 64:731-40; Johnson M. P. et al. (2003) J. Med. Chem., 46:3189-92; Marino M. J. et al. (2003) Proc. Natl. Acad. Sci. USA., 100:13668-73; Mitsukawa K. et al. (2005) Proc Natl Acad Sci USA 102(51):18712-7; Wilson J. et al. (2005) Neuropharmacology 49:278; for a review see Mutel V. (2002) Expert Opin. Ther. Patents, 12:1-8; Kew J. N. (2004) Pharmacol. Ther., 104(3):233-44; Johnson M. P. et al. (2004) Biochem. Soc. Trans., 32:881-7; recently Ritzen A., Mathiesen, J. M., and Thomsen C. (2005) Basic Clin. Pharmacol. Toxicol. 97:202-13).
In particular molecules have been described as mGluR4 positive allosteric modulators (Maj et al. (2003) Neuropharmacology 45:895-906; Mathiesen et al. (2003) British Journal of Pharmacology 138:1026-1030). It has been demonstrated that such molecules have been characterized in in vitro systems as well as in rat brain slices where they potentiated the effect of LAP-4 in inhibiting transmission at the striatopallidal synapse. These compounds do not activate the receptor by themselves (Marino et al. (2003) Proc. Nat. Acad. Sci. USA 100:13668-13673). Rather, they enable the receptor to produce a maximal response to a concentration of glutamate or the Group III orthosteric agonist L-AP4 which by itself induces a minimal response.
PHCCC, a positive allosteric modulator of mGluR4 not active on others mGluRs (Maj et al. (2003) Neuropharmacology 45:895-906), has been shown to be efficacious in animal models of Parkinson's disease thus representing a potential novel therapeutic approach for Parkinson's disease as well as for other motor disorders and disturbances (Marino et al. (2003) Proc. Nat. Acad. Sci. USA 100:13668-13673), neurodegeneration in Parkinson's disease (Marino et al. (2005) Curr. Topics Med. Chem. 5:885-895; Valenti et al. (2005) J. Pharmacol. Exp. Ther. 313:1296-1304; Vernon et al. (2005) Eur. J. Neurosci. 22:1799-1806, Battaglia et al. (2006) J. Neurosci. 26:7222-7229), and neurodegeneration in Alzheimer's disease or due to ischemic or traumatic insult (Maj et al. (2003) Neuropharmacology 45:895-906).
PHCCC also has been shown to be active in animal model of anxiety (Stachowicz et al. (2004) Eur J Pharmacol 498:153-156). Previously, ACPT-1 has been showed to produce a dose-dependent anti-conflict effect after intrahippocampal administration and anti-depressant-like effects in rats after intracerebroventricular administration (Tatarczynska et al., 2002)
Activation of mGluR4 receptors which are expressed in a and F cells in the islets of Langerhans inhibits glucagon secretion. Molecules which activate or potentiate agonist activity of these receptors may be an effective treatment for hyperglycemia, one of the symptoms of type 2 diabetes (Uehara et al. (2004) Diabetes 53:998-1006).
The β-chemokine RANTES is importantly involved in neuronal inflammation and has been implicated in the pathophysiology of multiple sclerosis. Activation of Group III mGluRs with L-AP4 reduced the synthesis and release RANTES in wild-type cultured astrocytes, whereas the ability of L-AP4 to inhibit RANTES was greatly decreased in astrocyte cultures from mGluR4 knockout mice (Besong et al. (2002) Journal of Neuroscience, 22:5403-5411). These data suggest that positive allosteric modulators of mGluR4 may be an effective treatment for neuroinflammatory disorders of the central nervous system, including multiple sclerosis and related disorders.
Two different variants of the mGluR4 receptor are expressed in taste tissues and may function as receptors for the umami taste sensation (Monastyrskaia et al. (1999) Br. J. Pharmacol. 128:1027-1034; Toyono et al. (2002) Arch. Histol. Cytol. 65:91-96). Thus positive allosteric modulators of mGluR4 may be useful as taste agents, flavour agents, flavour enhancing agents or food additives.
There are anatomical evidence that the majority of vagal afferents innervating gastric muscle express group III mGluRs (mGluR4, mGluR6, mGluR7 and mGluR8) and actively transport receptors to their peripheral endings (Page et al (2005) 128:402-10). Recently, it was shown that the activation of peripheral group III mGluRs inhibited vagal afferents mechanosensitivity in vitro which translates into reduced triggering of transient lower oesophagal sphincter relaxations and gastroesophageal reflux in vivo (Young et al (2008) Neuropharmacol 54:965-975). Labelling for mGluR4 and mGluR8 was abundant in gastric vagal afferents in the nodose ganglion, at their termination sites in the nucleus tractus solitarius and in gastric vagal motoneurons. These data suggest that positive allosteric modulators of mGluR4 may be an effective treatment for gastro-esophageal reflux disease (GERD) and lower esophageal disorders and gastro-intestinal disorders.
International patent publication WO2005/007096 describes mGluR4 receptor positive allosteric modulator useful, alone or in combination with a neuroleptic agent, for treating or preventing movement disorders. However, none of the specifically disclosed compounds are structurally related to the compounds of the invention.    (i) International patent publications WO2007/072090 and WO2006/167029 describe respectively thienopyridines as modulators of mGluR1 and mGluR5 and tricyclic compounds as mGluR1 antagonists.    (ii) International patent publication WO2007/001975 describes heterocyclic carboxamides as histamine H3 antagonists. International patent publication WO2004/087048 describes benzamides as modulators of metabotropic glutamate receptors.    (iii) International patent publication WO2006/074884 describes thiazole-4-carboxamides as mGluR5 receptor antagonists.    (iv) US patent publications US2006199960 and US2006199828 describe 3-aminopyridine-2-carboxamides and 3-aminopyrazine-2-carboxamides respectively as metabotropic glutamate receptor antagonists.    (v) International patent publication WO99/36416 describes furan-2-carboxamides and thiophen-2-carboxamides as modulators of metabotropic glutamate receptors.
It has now surprisingly been found that the compounds of general formula I show potent activity and selectivity on mGluR4 receptor. The compounds of the invention demonstrate advantageous properties over compounds of the prior art. Improvements have been observed in one or more of the following characteristics of the compounds of the invention: the potency on the target, the selectivity for the target, the bioavailability, the brain penetration, and the activity in behavioural models.
The present invention relates to a method of treating or preventing a condition in a mammal, including a human, the treatment or prevention of which is affected or facilitated by the neuromodulatory effect of mGluR4 modulators.